1. Field of the Invention
The present invention relates to a sensing device sensing a substance to be sensed by utilizing the fact that a natural frequency of a piezoelectric resonator changes when the substance to be sensed is adsorbed by an adsorption layer formed on the piezoelectric resonator.
2. Description of the Related Art
Conventionally, as a method of sensing a trace amount of a substance in, for example, a sample solution, there has been known a sensing device using a quartz-crystal resonator which is one of piezoelectric resonators. This sensing device captures antigens by an adsorption layer, for example, an antibody layer, formed on the quartz-crystal resonator by using an antigen-antibody reaction, and evaluates an amount of change in an oscillation frequency of the quartz-crystal resonator as an amount of the captured antigens, that is, as a concentration of the substance to be sensed in the sample solution.
A quartz crystal sometimes has useless distortion or an internal stress, and in this state, the oscillation frequency of the quartz-crystal resonator becomes unstable. Since the sensing device measures the substance to be sensed based on the oscillation frequency unique to the quartz-crystal resonator, the unstable oscillation frequency does not allow the high-precision sensing of the substance to be sensed.
The quartz-crystal resonator has a characteristic that, when it is oscillated, the distortion and internal stress of its quartz-crystal piece gradually disappear and accordingly its oscillation frequency becomes stable. When the quartz-crystal resonator is intensely excited by being supplied with high power equal to or more than twice regular power, preferably, equal to or more than five times regular power and is given a great mechanical vibration, the disappearance of the distortion and internal stress is promoted, so that the oscillation frequency of the quartz-crystal resonator stabilizes in a short time. Therefore, conventionally, before shipping, the quartz-crystal resonator is sometimes subjected to processing in which the quartz-crystal resonator is intensely excited by the supply of high power and thus is given a great mechanical vibration, thereby stabilizing the oscillation frequency of the quartz-crystal resonator. This method, however, has a problem that the effect weakens after two to three months pass.
Further, an oscillator circuit is not capable of supplying high power and not capable of causing the intense excitation of the quartz-crystal resonator since it generally uses a constant current circuit. Therefore, in a conventional sensing device, the oscillation frequency is stabilized by an aging process in which regular power is supplied to cause the gradual disappearance of the distortion and internal stress of a quartz crystal. This method, however, takes a longer time for stabilizing the oscillation frequency of the quartz-crystal resonator than the method of supplying the high power. This method has another problem of low work efficiency since the sensing of a substance to be sensed cannot be performed during this aging process. This has given rise to a demand for a sensing device in which an oscillation frequency can be stabilized in a shorter time.
A patent document 1 describes a holder for electrochemical sensor having a capacitor to which two electrode terminals of a biosensor are connected, and performing the aging after the measurement of the biosensor is finished, whereby an electrolyte surface holds highly stable electric charges and the electric charges are accumulated in the capacitor, so that the state where the electrolyte surface holds the highly stable electric charges is maintained. However, the patent document 1 aims at shortening the time taken for the preparation for the next measurement after the measurement is once finished, and does not give any description regarding the reduction in the time required for the preparation for the measurement when the biosensor is attached to the device. Further, the object of the aging in the patent document 1 is to maintain the state where the electrolyte surface holds the highly stable electric charges and is not the aforesaid disappearance of the distortion and stress of the quartz crystal.
Further, a patent document 2 describes a carbon monoxide sensor that includes a sensor element in which an alumina substrate, a platinum heater, an insulating layer, an oxide catalyst layer, a reference electrode, a solid electrolyte thick film, and a detecting electrode are stacked, and performs aging by applying a higher voltage than a regular voltage to the heater and applying a voltage across the electrodes upon power on, thereby lowering sensitivity to hydrogen gas and improving sensitivity to carbon monoxide. However, the object of the aging in the patent document 2 is to improve the sensitivity to carbon monoxide by applying the voltage and is not the aforesaid disappearance of the distortion and stress of the quartz crystal.
[Patent document 1] Japanese Patent Application Laid-open No. 2005-164407 (paragraph No. 0055, 0056)
[Patent document 2] Japanese Patent Application Laid-open No. Hei 11-304752 (paragraph No. 0023, 0024)